The present invention relates generally to voltage limiting compositions which are utilized as nonlinear resistors and particularly to a composition which displays specific operating characteristics and which is especially suitable for use in gapless surge arresters.
Nonlinear resistors, that is, resistors displaying nonlinear current-voltage characteristics, are used widely throughout the electronics industry in a number of different applications. As a result, these type of resistors differ substantially from one another in both physical attributes as well as operating characteristics, depending upon their ultimate intended use. However, all of these resistors in their nonlinear region of operation have a common nonlinear current-voltage characteristic which may be expressed by the empirical relation: EQU I=KV.sup..alpha.
where V is voltage across the resistor, I is the current flowing through the resistor, K is a constant related to the geometry and to the microstructure of the resistor and .alpha. is a nonlinear exponent having a value greater than 1. The value of .alpha. is calculated by the following equation: ##EQU1## where V.sub.1 and V.sub.2 are the voltages at given currents I.sub.1 and I.sub.2, respectively. The selection of these currents depends for the most part on the ultimate intended use of the resistor. For example, .alpha. may be determined over a current range of 0.1 ma (I.sub.1) to 1 ma (I.sub.2) or, in accordance with the present invention, as will be seen hereinafter, it is calculated over a much wider range, specifically over a range of 1 ma to 5000 amps.
To date, there has been a large number of studies on the relationship between the particular components making up a nonlinear exponent .alpha.. For example, in U.S. Pat. No. 3,760,318, it can be seen that .alpha., actually "n" in the patent, varies widely, depending upon the particular composition of the resistor. This is equally true in U.S. Pat. No. 3,764,566. Moreover, it can be seen that the constant K, actually "C" in the patent where K equals 1/C.sup.n is also varied. In both cases, one object is to provide a high .alpha. or n value at a readily controlled, preferably high K (low C) value. In the case of the '318 this is accomplished by diffusing lithium ions or sodium ions into a zinc oxide sintered body. In the case of the '566 patent, a resistor consisting of zinc oxide as its major constituent along with a number of other specific additives, for example silicon dioxide, bismuth oxide, cobalt oxide, manganese oxide and nickel oxide among others, is utilized to obtain a high .alpha. or n value and a high K or low C value, as well as a high degree of stability with respect to temperature, humidity, electric load and high resistance to surge current.
In addition to these patents, there are others which either seek to attain the same general objectives or which have other objectives in mind. For example, in U.S. Pat. No. 3,764,951, a nonlinear resistor having a stable voltage-current characteristic is the primary object and presumably this is accomplished by utilizing a sintered wafer consisting of iron oxide as the main component and calcium oxide as one of the number of additives combined with the iron oxide. In U.S. Pat. No. 3,570,002, one object is to provide a high .alpha. or n value and a high K or low C value. In this patent, the sintered disc of zinc oxide is combined with two electrodes, one being a silver electrode in non-ohmic contact with one surface of the disc and the other being the ohmic contact with an opposite surface. In U.S. Pat. No. 3,642,664, one of its objects is to provide a controllable .alpha. or K value, specifically a high value of .alpha.. Another object is to provide a resistor having what is referred to as negative resistance. In this particular patent, a composition utilizing zinc oxide as its major constituent is disclosed in combination with an additive selected from a particular group including manganese fluoride, magnesium fluoride, calcium fluoride, etc.
It should be apparent from the foregoing that a great number of different characteristics of a nonlinear resistor can be manipulated depending upon the particular combination of constituents making up the resistor. Certainly one such characteristic is the nonlinear exponent .alpha.. The resistor having a low .alpha., an intermediate .alpha. or a high .alpha. can readily be provided by those with ordinary skill in the art. In this regard it should be noted that a high .alpha. over a relatively small range, for example, 0.1 ma to 1 ma or a larger but relatively narrow range, for example 100a-1000a as in U.S. Pat. No. 3,838,378, can be attained. However, none of the patents cited disclosed a nonlinear resistor having a high .alpha. value over the current range of the present invention, specificallly from 1 ma to 5000 amps. This is an important feature, particularly where the nonlinear resistor is used in a gapless surge arrester. Moreover, it should be quite apparent that merely because a resistor has a high .alpha. over a low and/or narrow current range does not mean that the same .alpha. will be realized over the higher range of the present invention. In fact, .alpha. over a wide range will generally be less than over a narrow range.
What is seriously lacking in the prior art is a nonlinear resistor having both a high .alpha., particularly in the current range of the present invention, and a high surge energy absorption capability. Applicants have found that this particular combination of characteristics is quite important when the resistor is intended for use in high voltage surge arresters of the gapless type. As will be seen hereinafter, the present invention provides a composition having this unique combination.